The present invention is directed to method and apparatus for testing adhesion strength between two materials using a depth sensing indentation technique.
Thin films are very important in many applications. For example, thin films are used extensively in microelectronics applications where devices often have features of submicron size. Thin films are also used extensively in micro-mechanical applications for making devices such as microgears and accelerometers, and other applications such as for making hard disks in a hard drive and hard coating for gear boxes.
Determining the mechanical properties of thin films in these applications can be critical. For example, a thin film having a large tensile stress may delaminate causing device failure under certain conditions. The mechanical properties of a thin film material cannot be simply predicted based upon the properties of the bulk material for a number of reasons. The mechanical properties of the thin film will generally be different from that material in bulk form, and will depend upon the particular technique for forming the film, and the conditions under which the film is formed. For example, a thin film formed on a substrate at high temperature and then cooled to room temperature may exhibit either a tensile or compressive stress due to the difference in the coefficient of thermal expansion between the film and the substrate. Also a thin film may delaminate from a substrate due to stress applied by an outside source.
Depth sensing techniques, such as nanoindentaion and microindentation techniques, have been used for measuring material hardness and elastic modulus of a material. One exemplary system for performing nanoindentation hardness measurements is shown in FIG. 1. The apparatus 1 contains a sample stage 2 on which the sample to be tested 3 is placed. The indenter tip 4 is situated above the sample 3. As an example, the indenter tip may be a sharp Berkovich type diamond indenter tip which has a three sided pyramid tip with a known area to depth correlation. The indenter tip 4 is supported by the indentation column 5 which is moved up and down by a load application coil 6. The apparatus also contains indentation column guide springs 7 and a capacitive displacement sensor 8. The load application coil is connected to a current source 9 and an oscillator 10, while the capacitive displacement sensor is connected to a lock-in amplifier 11 and an electronic displacement sensor 12. The electronics are controlled by a computer 13.
The prior art apparatus operates in the following manner. The computer sends a signal to lower the indenter tip 4 into the sample. The computer operator enters into the computer a desired maximum load on the indenter tip. The indenter tip can penetrate up to a certain maximum depth into the sample for a given maximum load. Of course, for each material, the maximum depth differs for a given load because each material has a different hardness. The maximum penetration depth of the indenter tip is monitored by the capacitive displacement sensor 8. As the indenter tip 4 penetrates further into the sample, the capacitive plate attached to the indentation column 5 moves closer to the capacitive plate attached to the sample stage 2. Therefore, the capacitance between the two plates changes. The change in capacitance is detected by the electronic displacement sensor 12 which forwards the data to the computer 13. The computer than correlates the maximum load applied to the maximum penetration depth. The oscillator and lock-in amplifier may be used to scan the indenter tip across the sample.
Sample hardness may be calculated from nanoindentation measurements using, for example, two different methods. The first method is a depth measuring method. In this method, the penetration depth of the indenter tip for a predetermined peak (maximum) load is measured by the indentation apparatus and the contact area (e.g., the area of the sample contacted by the indenter tip) is extrapolated from the known shape and geometry of the indenter tip. Then sample hardness is calculated as a function of penetration depth.
The second method is an imaging method. In this method, the peak load exerted by the indentation apparatus is preset and the contact area is determined by an optical or electron microscopy examination. Sample hardness is determined from a ratio of the applied maximum load applied to the measured contact area.
Once hardness is calculated, the elastic modulus may be calculated from the hardness. The hardness may be calculated from a simple formula with a high error margin (See W. C. Oliver, R. Hutchins and J. B. Pethica, xe2x80x9cMeasurement of Hardness at Indentation Depths as Low as 20 nanometers,xe2x80x9d Nanoindentation Techniques in Materials Science and Engineering, ASTM STP 889, P. J. Blau et al. eds., ASTM (1986) pp. 90-108, incorporated herein by reference) or from finite element analysis calculations using the unloading portion of a load displacement curve with a low error margin (See W. C. Oliver and G. M. Pharr, J. Mater. Res. Vol. 7, No. 6 (1992) pp. 1564-83 and T. A. Laursen and J. C. Simo, J. Mater. Res. Vol. 7, No. 3(1992) pp. 618-26, both incorporated herein by reference). The sample may be a bulk sample or it may be a film on a substrate. Thus, hardness of a film may be measured by nanoindentation techniques.
Despite the different calculation techniques, hardness measurements in the prior art utilize a predetermined maximum load. In other words, the user selected a certain value of the maximum load, then this maximum load was applied to the sample, the penetration depth or contact area were measured, and hardness was calculated.
In addition, the prior art adhesion testing methods require destructive testing of the adhered materials. For example, in one prior art method, a representative article containing a film adhered to a substrate would be selected during mass production and pulled apart from the substrate by a layer of glue attached to the film. However, this stud pull method requires a large testing area and is expensive and time consuming. It also cannot be used to test the adhesion strength of all articles actually used or sold to the customers. Thus, even if adequate adhesion strength between materials was measured on the test article, other articles sold to customers or used by the manufacturer could easily have inadequate adhesion strength between adhered materials. Furthermore, the stud pull test cannot be properly used for thin films, such as those thinner than a thousand microns.
A second prior art method of adhesion testing is the blister test, where a hole is made in the substrate and then liquid or air pressure is applied through the hole to bulge the film outward. This method has the same disadvantage as the above-noted stud pull test. This method is also cumbersome and destructive.
A third prior art method of adhesion strength testing is the scratch test, where the film is scratched until it delaminates from the substrate. Such a method is taught, for example, by T. W. Wu et al., MRS Symp. Proc., Vol. 130 (1989) page 117, and in Adhesion Measurement of Thin Films, Thick Films and Bulk Coatings, K. L. Mittal, ed., ASTM (1978), pages 134-183, both incorporated herein by reference. In this method, a indenter tip is dragged across a thin film under an increasing load until the film delaminates from the substrate. The critical load for delamination is determined from an onset of a drop in the load. However, this method damages a significant portion of the film under investigation. This method also is subject to inaccuracies, because the drop in the load may be caused by flaking of the thin film instead of by delamination of the film from the substrate. Since for certain brittle thin films the onset of flaking precedes the delamination, the adhesion strength of such thin films cannot be accurately determined by the scratch test.
One object of an embodiment of the present invention is a nondestructive depth sensing indentation adhesion strength measurement method that avoids the above mentioned problems of destructive testing methods. The adhesion strength between parts of each article under production may be tested according to the method of an aspect of the current invention without the device being rendered inoperable.
Another object of an embodiment of the present invention is to form a film on a substrate and to determine its adhesion strength to the substrate by using a depth sensing indentation technique, such as nanoindentation.
A further object of an embodiment of the present invention is to utilize a depth sensing indentation apparatus, such as a nanoindentation apparatus, to measure a property of a material other than hardness or elastic modulus, such as adhesion strength of the material to a substrate.
Yet another object of an embodiment of the present invention is to perform in-situ measurement of adhesion strength of materials undergoing processing without requiring expensive and inconvenient ex-situ destructive testing.
Yet a further object of an embodiment of the present invention is an integrated test element formed on a substrate along with a thin film. The integrated test element may be used to test adhesion strength of the film to a substrate without damage to the film.
In accordance with an aspect of the present invention, there is provided a method of forming a thin film comprising forming a film on the substrate, forming a nanoindentation in at least the film and determining the adhesion strength of the film to the substrate. Furthermore, the film is rejected if the adhesion strength of the film to the substrate is below a desired value and the film and the substrate are processed further if the adhesion strength of the film to the substrate is equal to or above a desired value.
In accordance with another aspect of the present invention, there is provided a method of determining adhesion strength between two materials comprising forming a nanoindentation in one material and calculating the adhesion strength between the two materials. Furthermore, the first material is preferably a thin film and the second material is preferably a substrate.
In exemplary embodiments of the present invention, the depth sensing measurement is performed by pressing an indenter tip into the first material under a variable applied load while monitoring the applied load and penetration depth of the indenter tip. The load is increased with time and the indenter tip penetrates into the first material. The increasing load causes the indenter tip and the first material to xe2x80x9csink inxe2x80x9d the second material. The first material at least partially delaminates from the second material under the increasing load, and the indenter tip xe2x80x9csinks inxe2x80x9d the second material at a greater penetration depth rate than the penetration depth rate prior to the time the first material at least partially delaminates from the second material. An increase in the penetration depth rate and a critical value of the applied load at the time of the increase in the penetration depth rate are detected. A critical stress required to at least partially delaminate the first material from the second material is then calculated from the detected value of the critical value of the applied load. Finally, the adhesion strength between the first material and the second material is calculated from the critical stress.
In accordance with another aspect of the present invention, an apparatus for adhesion strength testing comprises an indenter tip, means for applying a variable load to the indenter tip, means for monitoring a penetration depth of the indenter tip into a sample, and means for calculating adhesion strength of a first part of the sample to a second part of the sample. Preferably, the apparatus also contains means for detecting a change in the penetration depth rate and means for detecting a critical value of the applied load at the time of the change in the penetration depth rate. In accordance with the present invention, the apparatus is used to calculate the adhesion strength of a first part of a sample to a second part of the sample.
In accordance with one embodiment of the present invention, there is provided a method of testing adhesion strength of a thin film to a substrate. The thin film and the substrate may comprise any of various materials. According to the present invention, the substrate may comprise either a base material or at least one intermediate film formed on a base material. The substrate may be at least one of a semiconductor device, an integrated circuit, a metal layer, an insulating layer, a gear box, a micromechanical device, a liquid crystal display, an electron emissive cathode substrate and a magnetic disk. A micromechanical device may comprise an air bag activation system or a solid state accelerometer.
According to the present invention, the thin film may comprise a single layer or a plurality of layers. The thin film may be at least one of metal film, an electrode for a semiconductor device, an interconnection for an integrated circuit or a liquid crystal display, a gear box coating, a magnetic disk overcoat, an electron emissive cathode and a micromechanical device portion. Preferably, the thin film thickness is 5 microns or less, and preferably, the thin film element being tested has a limited or finite width. The thin film test element can have any shape when viewed from the top, such as a linear strip, a rectangle, a square, a circle or an oval.
In accordance with another embodiment of the present invention, the thin film contains a first portion and a second portion, wherein the first portion is an integrated test element. Preferably, the integrated test element is not in physical contact with a second portion of the thin film. The integrated test element is used to test the adhesion strength of the film to the substrate during processing of the film without damaging the second portion of the film. The integrated test element contains a nanoindentation therein, and is preferably at least partially delaminated from the substrate after the adhesion strength testing is carried out.
In accordance with one aspect of this embodiment, the integrated test element is formed in an integrated circuit. The integrated test element allows in-situ adhesion strength testing of each layer of integrated circuit metallization to an underlying layer without removing the circuit wafer from a multichamber apparatus.
In accordance with another aspect of this embodiment, more than one thin film and more than one integrated circuit elements are formed. The plural integrated test elements allow adhesion strength testing of the first thin film to the substrate as well as the adhesion strength testing of the second thin film to the first thin film, and so on.
In accordance with another embodiment of the invention, sacrificial spacers are formed under the edges of a thin film test element. Removal of the sacrificial spacers speeds up delamination of the test element from the substrate during adhesion strength testing.
The term xe2x80x9cdepth sensing indentation technique,xe2x80x9d as used herein, refers not only to nanoindentation measurements, but also to various other depth sensing methods, such as Rockwell Hardness Testing method. The term xe2x80x9cnanoindentation,xe2x80x9d as used herein, includes microindentation within its scope, and refers to a relatively small indentation in a material, and is not meant to be limited to indentations that have dimensions measured in nanometers. Furthermore, nanoindentation is formed by lowering an indenter tip or stylus down into the sample without dragging or moving it across the surface of the sample, as is common in the scratch test.